Continuing the series, here is a look at other destinations in the solar system. I am still assuming the use of chemical propulsion, so this limits us to locations inside the orbit of Jupiter more or less. I will examine launches from EML1/2 and from Mars orbit. Most transit data is from Project Rho, so these values are somewhat pessimistic (as explained in the link).
I'll cover Venus, Apollo/Aten objects and Main Belt objects after the jump.
Venus is our bizarro-world sister planet. With no convenient moon for refueling this is an expensive place to visit. There are concepts for Venus that might make sense eventually (floating cities, permanent short-period cycler habitats, etc.). In the short term, think of it as an exploration and science destination. Delta-V for the trip is about 9.5km/s one-way from LEO (6.8 to 7.1 km/s from EML1/2), or 7.9km/s from Mars. Travel times are 4.8 months from Earth or 8.5 months from Mars. Synodic periods are 1 year 7 months from Earth and only 11 months from Mars. In many ways, Venus flyby is a less challenging life support problem than Mars flyby. The delta-V is fairly hefty, so pre-positioning fuel is essential to an affordable mission. A crewed orbital mission with a magazine of probes could do a lot to improve our knowledge of the atmosphere and surface conditions, though the advantages of crew on site are not likely to outweigh the added expense.
Asteroid 2000 BD19 (designation 137924) is the Aten-class asteroid with the smallest known perihelion (0.092 AU). A high percentage of potentially hazardous asteroids are from this class. Landing a probe on this body would allow for periodic close observation of the sun as well as a different viewing angle for other Apollo and Aten class objects. This is a very challenging environment with solar intensity nearly 120 times that of Earth normal (about 161 kW/m²). The asteroid itself is inclined almost 26°. Ideal transit in the next year has a departure C3 as low as 18 km²/s² (Vinf of 4.24km/s), for an EML2 launch dV of only 1.54km/s. I've not found a good solution for arrival dV, so I can't comment on the overall dV cost for the mission. Of course, anything that makes it is still going to cook.
Other bodies of the Aten or Apollo class would be interesting targets for similar reasons. If one is chosen with perihelion of perhaps 0.5 AU or more then a lander with a decent telescope could be sent and used to observe other members of the class. Right now most known members have fairly high inclination, but this is thought to be because observing low-inclination bodies close to the sun is very difficult; that is, sample bias. There could be a large number of undiscovered bodies with low inclination, and many of those will be potential Earth crossers (some of which will be potentially hazardous).
An alternative to rendezvous would be to place a pair of telescopes at Sun-Venus L4 and L5. These craft would allow us to identify and characterize most of the bodies crossing Venus orbit regardless of inclination. Different orbits are possible, but these would be stable over very long timescales while still allowing us to survey low-inclination bodies that are difficult to see from Earth. A number of other interesting astronomy tasks could be performed, including immediate parallax measurements to determine distance to objects in the outer solar system and beyond.
Ceres and other main belt objects like Pallas, Vesta, Juno, etc. are all at similar distances from the Sun. As a result the transit times, synodic period and delta-V requirements are similar. These bodies collectively are interesting for exploration because they offer us many slices of time in the early solar system and samples of material across nearly its whole diameter. They are commercially interesting because many have useful quantities of water and valuable metals. Ceres in particular appears to have useful quantities of nitrogen. All of the major bodies have minimal gravity, though generally high enough that current electric propulsion could not land or take off without chemical assistance.
Transit times are 1 year 1 month to 1 year 4 months from Earth or 1 year 4 months to 1 year 7 months from Mars. These journeys will require heavily-shielded spinning habitats similar to the cyclers for Mars crew. Mass is 12-15 tons per crewmember for the transit vehicle, then either 6 or 9 tons of supplies (for short vs. long stay). Obviously a robotic mission could get by without such amenities.
Synodic periods are 1 year 3 months to 1 year 4 months from Earth or 3 years 2 months to 3 years 4 months from Mars. This is a long time to wait, particularly for Mars, but if a permanent presence is established on several bodies then there will be frequent departures and arrivals at the two main hubs. Also worth noting is that transfer windows between the asteroids themselves are tens to hundreds of years apart, so all transport between asteroids will go to Earth or Mars first. Priority cargo will go direct to Earth to be relayed to another asteroid (between 2 years 5 months and 4 years total transit), while bulk cargo may be shipped to Mars for a bit less dV (between 2 years for perfect alignments and 5 years 11 months for worst cases).
Delta-V from low Earth orbit is 8.8 to 9.7 km/s or 6.1 to 7.3 km/s from EML1/2. From low Mars orbit, 4.0 to 5.1 km/s are required. These may vary a bit in either direction since some of the asteroids have significant inclination. Refueling at Ceres should be relatively simple, but we cannot be so sure about some of the other bodies. Still, it would be possible to send a robotic mining mission to a given target from Mars for perhaps 10km/s total dV and return with a cargo of partially-refined metals (for example) about every six years.